add domain options required to enable pipewire backend

[nyawox]

add options required to enable pipewire backend

example:

sound = {
  model = "ich9";
  codec = {type = "micro";};
  audio = {id = 1;};
};
audio = {
  id = 1;
  type = "pipewire";
  runtimeDir = "/run/user/1000"; # 1000 should be the default uid. couldn't find a way to avoid hardcoding
  input = {
    name = "qemuinput";
    streamName = "qemuinput";
    latency = 16384;
  };
  output = {
    name = "qemuoutput";
    streamName = "qemuoutput";
    latency = 16384;
  };
};

[AshleyYakeley]

Thanks!